Photosensitive semiconductor devices are used in many scientific and household devices for detection and processing of optical information. The key element of such devices is a photodiode that converts optical information into an electric signal. Photo-sensitivity and fast response time are basic working parameters of the photodiode. Conventionally, vacuum photo multipliers are used in such optical devices. However, semiconductor photoelectron multipliers or multi-pixel avalanche photodiodes (MAPD) (also named as multi-pixel photon counters (MPPC) or silicon photomultipliers (SiPM)) have also been developed which are an alternative to vacuum photoelectron multipliers.
For example, Russian patent 1702831 teaches a silicon substrate surface on which a matrix of small independent p-n-junctions (pixels) is formed. The residual surface of the substrate is filled by a dielectric layer (silicon dioxide). On the surface of both the pixels and the dielectric layer is formed a thin resistive layer of about 107 Ohm-cm resistivity and a semitransparent metal layer (field electrode). Avalanche amplification of photoelectrons is carried out in small dependent p-n-junctions (pixels). The avalanche current flows to the field electrode through the resistive layer fully covering the sensitive surface of pixels. However, this device is deficient because it provides low quantum efficiency in the visible spectrum because of low transparency of both the resistive layer and the field electrode.
U.S. Pat. No. 5,844,291 teaches a silicon substrate surface of n-type conductivity on which a resistive layer is disposed that comprises silicon carbide with certain resistivity, a dielectric layer, and an epitaxial silicon layer of p-type conductivity. Inside the dielectric layer, highly doped areas of n-type conductivity are formed having an electric contact with the resistive layer from one side, and with the epitaxial layer from another side. The photosensitive layer, in which photoelectrons are created, is an epitaxial layer grown on the surface of alien materials (i.e., dielectric and resistive layers). This device is also deficient because of the complexity of growing a silicon epitaxial layer on a dielectric surface.
Russian patent 2102820 teaches an MAPD device that comprises an array of small size p-n junctions (pixels) with characteristic sizes from 10 μm up to 100 μm formed on a semiconductor layer surface. The pixels are arranged at a certain spacing (about 10 μm) that is necessary to prevent charge coupling. Each pixel is connected to a common conductive grid by an individual micro resistor with a resistance of 105-106Ω. Due to low sizes of pixels, the MAPD may perform at overvoltage mode (i.e., above the breakdown potential). Then, the generation of a photoelectron (or a dark electron) in the sensitive region of a pixel the self-quenching avalanche process starts. This process is an analogous to the Geiger mode discharge.
The avalanche process is quenched when the potential on the pixel drops below the breakdown voltage due to the individual micro-resistor, which does not allow the pixel to be charged from the voltage source during the avalanche process. As a result, the unique combination of fast photo response (i.e., width of a pulse at half height of the amplitude which is about 10 ns) and high avalanche amplification of a signal (˜106) is achieved. The signals from operating pixels are added on the common conductive grid, which provides linearity of the MAPD photo response. The response remains linear as far as the probability for two or more photons to strike one pixel is insignificant.
However, some applied tasks need MAPD devices with faster photo response (about 1 ns) and larger sensitive area (at least 10 mm2 or more). Increasing the sensitive area leads to stretching its photo response because of high special capacity of the known MAPD devices. Additionally, high amplification of a photo signal in the known MAPD devices results in an undesirable effect—optical crosstalk. This effect is connected with high avalanche amplification (˜106) photo signals which are accompanied by additional emission of optical photons in avalanche region of the semiconductor. These photons are absorbed in the neighbor pixels of the MAPD device and cause false start of the avalanche process. In order to avoid the crosstalk effect, the avalanche amplification factor should be reduced below 105, but this low amplification factor is not enough for work in single photoelectron detection mode.
In view of the foregoing, a need exists for an improved multi-pixel avalanche photodiode (MAPD) systems and methods in an effort to overcome the aforementioned obstacles and deficiencies of conventional photosensitive semiconductor systems.
It should be noted that the figures are not drawn to scale and that elements of similar structures or functions are generally represented by like reference numerals for illustrative purposes throughout the FIGURES. It also should be noted that the figures are only intended to facilitate the description of the preferred embodiments. The FIGURES do not illustrate every aspect of the described embodiments and do not limit the scope of the present disclosure.